Effect of T(3)-induced hyperthyroidism on mitochondrial and cytoplasmic protein synthesis rates in oxidative and glycolytic tissues in rats

A tail of their own: regulation of cardiolipin and phosphatidylinositol fatty acyl profile by the acyltransferase LCLAT1

Cardiolipin and phosphatidylinositol along with the latter's phosphorylated derivative phosphoinositides, control a wide range of cellular functions from signal transduction, membrane traffic, mitochondrial function, cytoskeletal dynamics, and cell metabolism. An emerging dimension to these lipids is the specificity of their fatty acyl chains that is remarkably distinct from that of other glycerophospholipids. Cardiolipin and phosphatidylinositol undergo acyl remodeling involving the sequential actions of phospholipase A to hydrolyze acyl chains and key acyltransferases that re-acylate with specific acyl groups. LCLAT1 (also known as LYCAT, AGPAT8, LPLAT6, or ALCAT1) is an acyltransferase that contributes to specific acyl profiles for phosphatidylinositol, phosphoinositides, and cardiolipin. As such, perturbations of LCLAT1 lead to alterations in cardiolipin-dependent phenomena such as mitochondrial respiration and dynamics and phosphoinositide-dependent processes such as endocytic membrane traffic and receptor signaling. Here we examine the biochemical and cellular actions of LCLAT1, as well as the contribution of this acyltransferase to the development and specific diseases.

Assessing the Influence of Propylthiouracil and Phenytoin on the Metabolomes of the Thyroid, Liver, and Plasma in Rats

The thyroid hormones (THs) regulate various physiological mechanisms in mammals, such as cellular metabolism, cell structure, and membrane transport. The therapeutic drugs propylthiouracil (PTU) and phenytoin are known to induce hypothyroidism and decrease blood thyroid hormone levels. To analyze the impact of these two drugs on systemic metabolism, we focused on metabolic changes after treatment. Therefore, in a rat model, the metabolome of thyroid and liver tissue as well as from the blood plasma, after 2-week and 4-week administration of the drugs and after a following 2-week recovery phase, was investigated using targeted LC-MS/MS and GC-MS. Both drugs were tested at a low dose and a high dose. We observed decreases in THs plasma levels, and higher doses of the drugs were associated with a high decrease in TH levels. PTU administration had a more pronounced effect on TH levels than phenytoin. Both drugs had little or no influence on the metabolomes at low doses. Only PTU exhibited apparent metabolome alterations at high doses, especially concerning lipids. In plasma, acylcarnitines and triglycerides were detected at decreased levels than in the controls after 2- and 4-week exposure to the drug, while sphingomyelins and phosphatidylcholines were observed at increased levels. Interestingly, in the thyroid tissue, triglycerides were observed at increased concentrations in the 2-week exposure group to PTU, which was not observed in the 4-week exposure group and in the 4-week exposure group followed by the 2-week recovery group, suggesting an adaptation by the thyroid tissue. In the liver, no metabolites were found to have significantly changed. After the recovery phase, the thyroid, liver, and plasma metabolomic profiles showed little or no differences from the controls. In conclusion, although there were significant changes observed in several plasma metabolites in PTU/Phenytoin exposure groups, this study found that only PTU exposure led to adaptation-dependent changes in thyroid metabolites but did not affect hepatic metabolites.

Effects of Thyroid Hormone Therapy on Cut-Surface Healing of the Remnant Stomach with Short-Term Weight Loss Alterations after Sleeve Gastrectomy

BACKGROUND

The hypothalamic-pituitary-tyhroid axis is directly affected by drastic changes in energy stores. The aim of the present study was to determine the effects of triiodothyronine (T3) treatment on cut-surface healing of remnant stomach with weight loss alterations after sleeve gastrectomy (SG).

METHODS

Thirty male Wistar Albino rats were divided into three groups: sham (n = 6), control (n = 12), and experimental (n = 12). Control and experimental group rats underwent sleeve gastrectomy. Experimental group rats received a single dose of T3 (400 mg/100 g) on the first postoperative day whereas control group rats received 0.9% NaCl. All rats were sacrificed on the seventh postoperative day.

RESULTS

In the group of rats receiving T3, levels of FT3 were significantly higher and that of FT4 were significantly lower compared with both the control and sham group rats (p <.05). No significant difference was found between control and T3 group rats in terms of weight loss (p >.05). Microscopic examination of the cut surface of remnant stomach in the control group rats revealed significantly more severe tissue necrosis, edema, and disruption of mucosal epithelium than in the T3 group rats (p <.05). On the other hand, bridging of the submucosal and muscular layers, tissue granulation, fibroblast accumulation, neoangiogenesis, and collagen deposition in the T3 group rats were significantly higher than in the control group rats (p <.05).

CONCLUSIONS

Sleeve gastrectomy did not significantly alter thyroid hormone levels in short term. T3 hormone therapy seems to deliver constructive therapeutic effects for wound healing while causing no adverse effect on weight reduction.

Thyroid hormone activation by type 2 deiodinase mediates exercise-induced peroxisome proliferator-activated receptor-γ coactivator-1α expression in skeletal muscle

KEY POINTS

In skeletal muscle, physical exercise and thyroid hormone mediate the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1a) expression that is crucial to skeletal muscle mitochondrial function. The expression of type 2 deiodinase (D2), which activates thyroid hormone in skeletal muscle is upregulated by acute treadmill exercise through a β-adrenergic receptor-dependent mechanism. Pharmacological block of D2 or disruption of the Dio2 gene in skeletal muscle fibres impaired acute exercise-induced PGC-1a expression. Dio2 disruption also impaired muscle PGC-1a expression and mitochondrial citrate synthase activity in chronically exercised mice.

ABSTRACT

Thyroid hormone promotes expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1a), which mediates mitochondrial biogenesis and oxidative capacity in skeletal muscle (SKM). Skeletal myocytes express the type 2 deiodinase (D2), which generates 3,5,3'-triiodothyronine (T3 ), the active thyroid hormone. To test whether D2-generated T3 plays a role in exercise-induced PGC-1a expression, male rats and mice with SKM-specific Dio2 inactivation (SKM-D2KO or MYF5-D2KO) were studied. An acute treadmill exercise session (20 min at 70-75% of maximal aerobic capacity) increased D2 expression/activity (1.5- to 2.7-fold) as well as PGC-1a mRNA levels (1.5- to 5-fold) in rat soleus muscle and white gastrocnemius muscle and in mouse soleus muscle, which was prevented by pretreatment with 1 mg (100 g body weight)(-1) propranolol or 6 mg (100 g body weight)(-1) iopanoic acid (5.9- vs. 2.8-fold; P < 0.05), which blocks D2 activity . In the SKM-D2KO mice, acute treadmill exercise failed to induce PGC-1a fully in soleus muscle (1.9- vs. 2.8-fold; P < 0.05), and in primary SKM-D2KO myocytes there was only a limited PGC-1a response to 1 μm forskolin (2.2- vs. 1.3-fold; P < 0.05). Chronic exercise training (6 weeks) increased soleus muscle PGC-1a mRNA levels (∼25%) and the mitochondrial enzyme citrate synthase (∼20%). In contrast, PGC-1a expression did not change and citrate synthase decreased by ∼30% in SKM-D2KO mice. The soleus muscle PGC-1a response to chronic exercise was also blunted in MYF5-D2KO mice. In conclusion, acute treadmill exercise increases SKM D2 expression through a β-adrenergic receptor-dependent mechanism. The accelerated conversion of T4 to T3 within myocytes mediates part of the PGC-1a induction by treadmill exercise and its downstream effects on mitochondrial function.

Thyroid Hormone Stimulation of Autophagy Is Essential for Mitochondrial Biogenesis and Activity in Skeletal Muscle

Thyroid hormone (TH) and autophagy share similar functions in regulating skeletal muscle growth, regeneration, and differentiation. Although TH recently has been shown to increase autophagy in liver, the regulation and role of autophagy by this hormone in skeletal muscle is not known. Here, using both in vitro and in vivo models, we demonstrated that TH induces autophagy in a dose- and time-dependent manner in skeletal muscle. TH induction of autophagy involved reactive oxygen species (ROS) stimulation of 5'adenosine monophosphate-activated protein kinase (AMPK)-Mammalian target of rapamycin (mTOR)-Unc-51-like kinase 1 (Ulk1) signaling. TH also increased mRNA and protein expression of key autophagy genes, microtubule-associated protein light chain 3 (LC3), Sequestosome 1 (p62), and Ulk1, as well as genes that modulated autophagy and Forkhead box O (FOXO) 1/3a. TH increased mitochondrial protein synthesis and number as well as basal mitochondrial O2 consumption, ATP turnover, and maximal respiratory capacity. Surprisingly, mitochondrial activity and biogenesis were blunted when autophagy was blocked in muscle cells by Autophagy-related gene (Atg)5 short hairpin RNA (shRNA). Induction of ROS and 5'adenosine monophosphate-activated protein kinase (AMPK) by TH played a significant role in the up-regulation of Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PPARGC1A), the key regulator of mitochondrial synthesis. In summary, our findings showed that TH-mediated autophagy was essential for stimulation of mitochondrial biogenesis and activity in skeletal muscle. Moreover, autophagy and mitochondrial biogenesis were coupled in skeletal muscle via TH induction of mitochondrial activity and ROS generation.

Altered regulation of energy homeostasis in older rats in response to thyroid hormone administration

Hyperthyroidism causes increased energy intake and expenditure, although anorexia and higher weight loss have been reported in elderly individuals with hyperthyroidism. To determine the effect of age on energy homeostasis in response to experimental hyperthyroidism, we administered 200 μg tri-iodothyronine (T3) in 7- and 27-mo-old rats for 14 d. T3 increased energy expenditure (EE) in both the young and the old rats, although the old rats lost more weight (147 g) than the young rats (58 g) because of the discordant effect of T3 on food intake, with a 40% increase in the young rats, but a 40% decrease in the old ones. The increased food intake in the young rats corresponded with a T3-mediated increase in the appetite-regulating proteins agouti-related peptide, neuropeptide Y, and uncoupling protein 2 in the hypothalamus, but no increase occurred in the old rats. Evidence of mitochondrial biogenesis in response to T3 was similar in the soleus muscle and heart of the young and old animals, but less consistent in old plantaris muscle and liver. Despite the comparable increase in EE, T3's effect on mitochondrial function was modulated by age in a tissue-specific manner. We conclude that older rats lack compensatory mechanisms to increase caloric intake in response to a T3-induced increase in EE, demonstrating a detrimental effect of age on energy homeostasis.

Skeletal muscle mitochondrial energetics in obesity and type 2 diabetes mellitus: endocrine aspects

During the development of type 2 diabetes mellitus, skeletal muscle is a major site of insulin resistance. The latter has been linked to mitochondrial dysfunction and impaired fatty acid oxidation. Some hormones like insulin, thyroid hormones and adipokines (e.g., leptin, adiponectin) have positive effects on muscle mitochondrial bioenergetics through their direct or indirect effects on mitochondrial biogenesis, mitochondrial protein expression, mitochondrial enzyme activities and/or AMPK pathway activation--all of which can improve fatty acid oxidation. It is therefore not surprising that treatment with these hormones has been proposed to improve muscle and whole body insulin sensitivity. However, treatment of diabetic patients with leptin and adiponectin has no effect on muscle mitochondrial bioenergetics showing resistance to these hormones during type 2 diabetes. Furthermore, treatment with most thyroid hormones has unexpectedly revealed negative effects on muscle insulin sensitivity. Future research should focus on development of agents that improve metabolic dysfunction downstream of hormone receptors.

Effects of thyroid hormone supplementation on anastomotic healing after segmental colonic resection

BACKGROUND

Alterations of thyroid hormones in colorectal surgery were previously studied. The aim of the present study was to determine the effects of triiodothyronine (T3) supplementation on anastomotic healing after segmental colectomy.

MATERIAL AND METHODS

Thirty male Wistar albino rats were divided into sham (n = 6), control (n = 12), and experimental (n = 12) groups. Sham group rats were immediately sacrificed after segmental colonic resection. Control and experimental group rats underwent resection and anastomosis. Experimental group rats received a single dose of T3 (400 μg/100 g) in postoperative day 1. Half of both control and experimental group rats were sacrificed on postoperative d 3 and the remaining half were sacrificed on postoperative d 7. Hydroxiproline (HP), myeloperoxidase (MPO), thyroid stimulating hormone (TSH), free T3 (FT3), and free thyroxine (FT4) levels, bursting pressure, and histologic analyses of the anastomotic segments were compared.

RESULTS

FT3 levels significantly decreased in control groups rats compared with the sham group (P < 0.01). However, T3 hormone given rats had no decline in FT3 levels. Anastomotic bursting pressure was significantly higher in the experimental group rats on postoperative d 7 (P = 0.015). Histopathologic analyses of the anastomotic segments determined significantly more severe edema and necrosis in control group rats (P < 0.05). Collagen deposition in the anastomotic tissue was significantly higher in experimental group rats on postoperative d 7 (P = 0.015).

CONCLUSION

Anastomosis after colon resection is associated with decreased FT3 level. T3 supplementation ameliorates the reduction in FT3 and seems to provide constructive therapeutic effects on anastomotic healing.

Mitochondrial DNA transcription regulation and nucleoid organization

Mitochondrial biogenesis is a complex process depending on both nuclear and mitochondrial DNA (mtDNA) transcription regulation to tightly coordinate mitochondrial levels and the cell's energy demand. The energy requirements for a cell to support its metabolic function can change in response to varying physiological conditions, such as, proliferation and differentiation. Therefore, mitochondrial transcription regulation is constantly being modulated in order to establish efficient mitochondrial oxidative metabolism and proper cellular function. The aim of this article is to review the function of major protein factors that are directly involved in the process of mtDNA transcription regulation, as well as, the importance of mitochondrial nucleoid structure and its influence on mtDNA segregation and transcription regulation. Here, we discuss the current knowledge on the molecular mode of action of transcription factors comprising the mitochondrial transcriptional machinery, as well as the action of nuclear receptors on regulatory regions of the mtDNA.

Can dietary nitrates enhance the efficiency of mitochondria?

A decline in mitochondrial function occurs in many conditions. A report in this issue of Cell Metabolism (Larsen et al., 2011) shows that dietary inorganic nitrates enhance muscle mitochondrial efficiency. It is an attractive hypothesis that dietary changes enhance energy efficiency, but its potential application depends on long-term studies investigating net benefits versus adverse effects.

Insulin action in hyperthyroidism: a focus on muscle and adipose tissue

Hyperthyroidism leads to an enhanced demand for glucose, which is primarily provided by increased rates of hepatic glucose production due to increased gluconeogenesis (in the fasting state) and increased Cori cycle activity (in the late postprandial and fasting state). Adipose tissue lipolysis is increased in the fasting state, resulting in increased production of glycerol and nonesterified fatty acids. Under these conditions, increased glycerol generated by lipolysis and increased amino acids generated by proteolysis are used as substrates for gluconeogenesis. Increased nonesterified fatty acid levels are necessary to stimulate gluconeogenesis and provide substrate for oxidation in other tissues (such as muscle). In the postprandial period, insulin-stimulated glucose uptake by the skeletal muscle has been found to be normal or increased, mainly due to increased blood flow. Under hyperthyroid conditions, insulin-stimulated rates of glycogen synthesis in skeletal muscle are decreased, whereas there is a preferential increase in the rates of lactate formation vs. glucose oxidation leading to increased Cori cycle activity. In hyperthyroidism, the Cori cycle could be considered as a large substrate cycle; by maintaining a high flux through it, a dynamic buffer of glucose and lactate is provided, which can be used by other tissues as required. Moreover, lipolysis is rapidly suppressed to normal after the meal to facilitate the disposal of glucose by the insulin-resistant muscle. This ensures the preferential use of glucose when available and helps to preserve fat stores.

Cardiolipin remodeling by ALCAT1 links oxidative stress and mitochondrial dysfunction to obesity

Oxidative stress causes mitochondrial dysfunction and metabolic complications through unknown mechanisms. Cardiolipin (CL) is a key mitochondrial phospholipid required for oxidative phosphorylation. Oxidative damage to CL from pathological remodeling is implicated in the etiology of mitochondrial dysfunction commonly associated with diabetes, obesity, and other metabolic diseases. Here, we show that ALCAT1, a lyso-CL acyltransferase upregulated by oxidative stress and diet-induced obesity (DIO), catalyzes the synthesis of CL species that are highly sensitive to oxidative damage, leading to mitochondrial dysfunction, ROS production, and insulin resistance. These metabolic disorders were reminiscent of those observed in type 2 diabetes and were reversed by rosiglitazone treatment. Consequently, ALCAT1 deficiency prevented the onset of DIO and significantly improved mitochondrial complex I activity, lipid oxidation, and insulin signaling in ALCAT1(-/-) mice. Collectively, these findings identify a key role of ALCAT1 in regulating CL remodeling, mitochondrial dysfunction, and susceptibility to DIO.

Regulation of skeletal muscle mitochondrial function: genes to proteins

The impact of ageing on mitochondrial function and the deterministic role of mitochondria on senescence continue to be topics of vigorous debate. Many studies report that skeletal muscle mitochondrial content and function are reduced with ageing and metabolic diseases associated with insulin resistance. However, an accumulating body of literature suggests that physical inactivity typical of ageing may be a more important determinant of mitochondrial function than chronological age, per se. Reports of age-related declines in mitochondrial function have spawned a vast body of literature devoted to understanding the underlying mechanisms. These mechanisms include decreased abundance of mtDNA, reduced mRNA levels, as well as decreased synthesis and expression of mitochondrial proteins, ultimately resulting in decreased function of the whole organelle. Effective therapies to prevent, reverse or delay the onset of the aforementioned mitochondrial changes, regardless of their inevitability or precise underlying causes, require an intimate understanding of the processes that regulate mitochondrial biogenesis, which necessitates the coordinated regulation of nuclear and mitochondrial genomes. Herein we review the current thinking on regulation of mitochondrial biogenesis by transcription factors and transcriptional co-activators and the role of hormones and exercise in initiating this process. We review how exercise may help preserve mitochondrial content and functionality across the lifespan, and how physical inactivity is emerging as a major determinant of many age-associated changes at the level of the mitochondrion. We also review evidence that some mitochondrial changes with ageing are independent of exercise or physical activity and appear to be inevitable consequences of old age.

The effect of high glucocorticoid administration and food restriction on rodent skeletal muscle mitochondrial function and protein metabolism

Background

Glucocorticoids levels are high in catabolic conditions but it is unclear how much of the catabolic effects are due to negative energy balance versus glucocorticoids and whether there are distinct effects on metabolism and functions of specific muscle proteins.

Methodology/Principal Findings

We determined whether 14 days of high dose methylprednisolone (MPred, 4 mg/kg/d) Vs food restriction (FR, food intake matched to MPred) in rats had different effects on muscle mitochondrial function and protein fractional synthesis rates (FSR). Lower weight loss (15%) occurred in FR than in MPred (30%) rats, while a 15% increase occurred saline-treated Controls. The per cent muscle loss was significantly greater for MPred than FR. Mitochondrial protein FSR in MPred rats was lower in soleus (51 and 43%, respectively) and plantaris (25 and 55%) than in FR, while similar decline in protein FSR of the mixed, sarcoplasmic, and myosin heavy chain occurred. Mitochondrial enzymatic activity and ATP production were unchanged in soleus while in plantaris cytochrome c oxidase activity was lower in FR than Control, and ATP production rate with pyruvate + malate in MPred plantaris was 28% lower in MPred. Branched-chain amino acid catabolic enzyme activities were higher in both FR and MPred rats indicating enhanced amino acid oxidation capacity.

Conclusion/Significance

MPred and FR had little impact on mitochondrial function but reduction in muscle protein synthesis occurred in MPred that could be explained on the basis of reduced food intake. A greater decline in proteolysis may explain lesser muscle loss in FR than in MPred rats.